The field of the invention is in the liquid level sensing art.
The level of the liquid nitrogen in cryogenic sampling systems which are used to measure the amount of gas vapors in the atmosphere such as a jet and other fuel storage facilities is frequently quite critical. Prior art devices have generally been quite sensitive to variations in the input line voltage with resulting erratic operation. They generally use electromechanical relays, and the environment in which the equipment is used is generally not condusive to very long life of the metallic contacts. Many of the prior art devices require elaborate adjustments during operation and cannot be turned off during a cycle of operation without affecting future operation. The following U.S. patents are concerned with liquid level sensing and control and may be of interest in developing the background of the present invention: Thiele, No. 3,222,578; Chaney, No. 3,262,280; Thiele, No. 3,325,135; Rahn et al, No. 3,757,317; Byers et al, No. 3,797,514; Kachuk et al, No. 3,922,564; and Riedel et al, No. 3,938,347.